Spherical silver nanoparticles are readily synthesized. It is thought that spherical nanoparticles are more easily formed than elongated filaments or wires of similar scale due to the low surface energy to volume ratio afforded by the spherical shape. There are numerous published accounts of synthetic routes for making silver nanoparticles including the description in U.S. Patent Application Publication Number 2007/0003603 for “Antimicrobial Silver Compositions” published Jan. 4, 2007 and U.S. Patent Application Publication Number 2007/0207335 for “Methods and Compositions For Metal Nanoparticle Treated Surfaces” published Sep. 6, 2007. Elongated nanostructures such as nanowires, nanofilaments and nanorods, however, are less common in the literature.
Production of silver nanofilaments such as nanowires can be divided in to template-assisted and template-free methods. Template-assisted methods utilize a template such as, for example, carbon nanotubes, porous TiO2 or similar structure that will induce silver to grow into nanometer scale filaments or wires. Features such as diameter and length can be controlled by carefully selecting the appropriate template. However, template-assisted methods yield silver structures that are impure because of the difficulty of separating the silver from the template. In addition, template-assisted methods generally have low efficiencies and require hours or days for synthesis.
Template-free methods generally involve reducing the silver metal nanometer scale filament or wire from silver salt at elevated temperatures (e.g., about 80° C. to about 150° C.). Features such as diameter and length can be controlled by adjusting the amount of surfactant or by carefully selecting the reducing agent. Template-free methods generally have low efficiencies and typically require hours or even days to obtain milligram levels of output from the synthesis.
Microwave synthesis of silver nanometer scale wires using a template-free method is described in the literature. However, wire formation is highly dependent on the microwave heating power, time and ratios of certain ingredients.
Exemplary methods of producing nanometer scale wires may be found in, for example, U.S. Patent Application Publication No. 2005/0056118; U.S. Patent Application Publication No. 2007/0074316; U.S. Patent Application Publication No. 2009/0311530; U.S. Patent Application Publication No. 2009/0196788; U.S. Patent Application Publication No. 2010/0148132; J. Phys. Chem. B, 2005, 109, 8379; Journal of Solid State Chemistry, Volume 179, 2009, p. 696; Chemistry of Materials 2007, 19, 1755; and Nano Letters Vol. 3, No. 5, 667.
Although many methods for producing silver nanofilaments or nanowires have been proposed in these documents and the like, there still has been a demand for producing such nanofilaments or nanowires simply, efficiently, and cost effectively in a short time without using an aqueous solvent, pressurization by an autoclave or the like. Moreover, it is desired to prevent silver nanofilaments or nanowires from being oxidized.
Accordingly, there is a need for a simple method of producing silver nanofilaments or nanowires. For example, there is a need for a simple method of producing silver nanofilaments or nanowires without requiring days or hours to carry out the synthesis. There is also a need for a simple method of producing silver nanofilaments or nanowires that is efficient and easy to control.